1. Field of the Invention
The invention relates to the field of microelectronics and in particular to the fabrication of microcircuits on porous substrates with high resolution and the structures fabricated thereby.
2. Description of the Prior Art
Interest in porous materials has mainly been because of its altered material properties in comparison to compact matter. Examples are nano- or aerogel thermal insulators or in micro devices using porous silicon membranes. Porosity may also lead to lowered dielectric constants of matter or changes in electrical resistivity. If the pores are well ordered like in anodized alumina, they could be additionally used in functional structures where the pores serve as line connections between various levels of a three dimensional structure.
Micromachining bears some problems in the case of large scale precision machining of porous matter, because the widely used technique of resist lithography does not work on porous materials. The resist is sucked into the inner pores where dissolution is later hindered or difficult to achieve. Alternative strategies which have been used to surmount this problem are methods which deposit the porous material through a hard mask, such as in metal-oxide gas sensors, or which depose the porous material on top of an already patterned resist prepared for lift off.
For those layers, which have to be made porous after depositing a bulk material, this starting layer may be structured before processing. Etching the starting material or partially covering it with an anodizing-resistant layer may do this. However, these techniques do not provide parallel pores at the edges of the porous layer, as the processes are isotropic. As a rule of thumb the width of edge disordering will be equal to the starting material thickness.
Therefore, if ordering of pores will be essentially required, the present state of the art of structuring in porous material leads to large minimum feature sizes since the isotropic porous edge region cannot be used. A new method of patterning porous material is needed which maintains pore ordering and which makes feature sizes down to the inter-pore distance possible. From this, new applications of porous layers will arise.